Advances in recent decades suggest that we may have found an important and marvelous clue or “belief” to the mystery between gravity and quantum mechanics, namely “It from Qubit" [1]}, [2]}, [3]}, [4]}. In this view, spacetime is not a fundamental object, but rather emerges from a structure of quantum entanglement. The clue is successively related to the concepts of the black hole area entropy [5]}, [6]}, [7]}, [8]}, the holographic principle (especially AdS/CFT duality) [9]}, [10]}, [11]}, and the RT formula of the holographic entanglement entropy [12]}, [13]}, [14]}, which have built a bridge between quantum mechanics and general relativity. In particular, the RT formula shows that the entanglement entropy that characterizes quantum entanglement between different parts of a particular class of (i.e., “holographic") quantum systems can be equivalently (i.e., “dually") described by the area of an extremal surface in a corresponding curved spacetime [12]}, [13]}, [14]}.
